The demand for a problem-free, rapid and cost-effective detection method for substances, especially for harmful substances, has increased recently, one of the reasons being the increased demands for environmental protection. In order to comply with harmful-substance limiting values (for example, in controlling and reducing emissions) it is necessary to be able to detect harmful substance concentrations in gaseous or liquid media reliably and continuously, possibly by the use of electrochemical sensors. In process monitoring, too, the control of the concentration of products, starting materials and impurities may be necessary for optimum performance of the process. However, rapid detection is made difficult by the fact that the materials or substances are frequently slow to react.
Frequently, electrochemical detection is based on the amperometric principle and is aimed at the quantitative detection of a material component.
For this purpose, substances (Cl.sub.2, HCl, SO.sub.2, NO.sub.x, H.sub.2 CO, etc.) are reacted electrochemically, that is, by an oxidation or reduction reaction on a sensor electrode to which a constant potential is applied.
The current flowing can be set in relation to the concentration of the substance to be detected. The selectivity of a sensor electrode based on this working principle is limited by the electrode material used and by the potential that can be applied to the electrode. The potential that can be applied is limited to a range of values at which the oxygen of the air is not reduced and/or the electrolyte for the substance to be detected is not decomposed. Namely, the currents produced by these perturbing effects would overlap the actual measured signal almost completely. Moreover, some substances are not sufficiently reacted in the available potential range or poison the sensor electrode by adsorption, so that they cannot be detected by this method. These substances include many unsaturated compounds, halogenated hydrocarbons and aromatics.
Qualitative and quantitative electrochemical detection can be achieved by voltametric techniques. Here, the substance to be detected is not reacted at the electrode at a fixed potential. Rather, oxidation or reduction of the substance is catalyzed successively while a continuously varying potential is applied. The recorded relationship between the amount of charge passing through or current and applied potential can be correlated with the quantity and also with the nature of the substance to be detected.
In another electrochemical detection method, called the alternating current method, an alternating voltage is superimposed on the voltage applied to an electrode. The alternating current flowing through is measured. The measured alternating current is shifted in phase with respect to the applied alternating potential, namely, because of the electrode capacitance, which is changed by the adsorption of the substance to be detected and also because of oxidation and reduction processes. Therefore, a complex resistance is defined, which is called impedance below, which describes the processes on the electrode surface appropriately. Its frequency-dependent and potential-dependent real and imaginary parts give information about the concentration of the substance to be detected.
A detection method, which is similar to the alternating-current method is tensametry, known from the analysis for solutions (see for example Nurnberg et al., in Methodicum Chimicum, Volume 1/1, Stuttgart 1973). However, in such methods, the sample with the substance to be detected must always "be prepared" manually to some extent, that is, interfering impurities must be removed and the oxygen of the air must be excluded.
Thus, continuous detection cannot be performed with these two known methods--alternating-current method and tensametry.
In this connection, the determination of concentration of blood glucose is also known (Kasapbasioglu et. al., Sensors and Actuators B, 13-14 (1993), p. 749). Here, glucose is oxidized directly electrochemically to gluconic acid on a membrane-covered electrode made of a noble metal. The electrode functions as an electrocatalyst, to which a potential program that decreases and increases stepwise is applied. At each step, an alternating potential with a high frequency and one with a low frequency are superimposed onto the potential. The glucose concentration in the blood is determined from the resulting real and imaginary part of the impedance at certain potential steps.
Furthermore, it is known that the selectivity and sensitivity of an electrode can be increased in an electrochemical detection method by utilizing the adsorption or absorption of the substance to be detected on the electrode surface. The adsorption or absorption can be supported, weakened or eliminated by the applied potential or potential program. The substance to be detected is adsorbed at a potential at which the substance is not electro-chemically active. The amount adsorbed as a function of time is then correlated with the concentration of the substance to be detected.
A method is known from the technical journal "Sensors and Actuators B", Ege et al., 4 (1991), p. 519, with which the reactive carbon monoxide CO in a CO/H.sub.2 mixture can be detected quantitatively based on the amperometric principle. For the detection, first the carbon monoxide component is adsorbed on a platinum electrode and then reacted electro-chemically. The carbon monoxide is adsorbed specifically at a potential at which it is not electrochemically active or is not reacted. After adsorption of the carbon monoxide to the saturation value, the potential is increased to a value at which the carbon monoxide is oxidized. The amount of charge flowing during oxidation is measured and is integrated over the oxidation time. The measured signal thus obtained is correlated with the concentration of the carbon monoxide. However, the amount of charge flowing is additionally superimposed by amounts of charge stemming from the electrochemical reaction of additionally adsorbed substances, such as oxygen. This additional amount of flowing current is determined in another reference cycle, in order to correct the measured signal. In this reference cycle, a potential is applied over a very short period of time to adsorb the additionally adsorbed substances. The time period is made so short that the carbon monoxide is not adsorbed on the electrode surface of the sensor. Then the potential is brought to a suitable value for the electrochemical reaction of these additionally adsorbed substances. The amount of charge flowing during this electrochemical reaction is used as correction value, because it is influenced only by the additionally adsorbed substances. Minimum CO concentrations up to 0.05% CO can be detected in a CO/H.sub.2 mixture. However, this known method cannot provide continuous detection either. Moreover, there is no suitable sensor for the commercial utilization of this method of detection.
Similarly, the reactive carbon dioxide, CO.sub.2, can be detected quantitatively in air at concentrations from 5% to 0.3% CO.sub.2 (Kuver et al.; J. Electroanal. Chem., 353 (1993), p. 255).
It is also known that several substances can be detected quantitatively simultaneously with the aid of a chain of electrodes which mostly consist of different electrode materials. Different potentials are applied to the individual electrodes and one substance reacts electrochemically at each of these potentials. The measured signals obtained at the individual electrodes are correlated with the individual substance concentrations using pattern recognition technology.
The electrochemical detection methods mentioned above are not suitable for rapid, continuous, both qualitative as well as quantitative detection, or are very expensive. Moreover, substances with low reactivity cannot be detected with these known detection methods or can only be detected at high concentrations.
In general, known electrochemical detection methods are characterized by the fact that the selectivity is frequently too low. Similarly, most of the sensors based on these detection methods do not satisfy the general criteria of a sensor: The detection should occur rapidly and continuously without preparation of the sample "on location" with a time constant of the order of one or at most a few minutes. In addition, the sensor should operate in the ambient atmosphere, that is, generally in the presence of the oxygen of air and should also be cost-effective.
The goal of the invention is to provide further methods and devices for continuous and quantitative as well as qualitative detection of substances in gaseous or liquid mixtures.